This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-292298, filed Sep. 26, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to an on-vehicle planar antenna device for receiving satellite broadcasting.
Conventionally, there has been no technique but decreasing an antenna element size when a planar antenna device is used for obtaining high electromagnetic field radiation characteristics within the range of a wide elevation angle.
FIG. 1 illustrates a structure of a general air patch antenna device. FIG. 1 shows a ground plane 11, an antenna element 12 mounted on the ground plane 11 separated by a spacer 13, and a feed point 14 to the antenna element 12.
A microstrip antenna device stationed in the air (xcex5r=1) has a high relative antenna device gain. On the other hand, however, the half-power angle generally becomes approximately 60xc2x0 to 80xc2x0 depending on antenna device shapes. Consequently, a gain remarkably decreases toward a low elevation angle.
To decrease the antenna element size for widening such a narrow elevation angle range, a dielectric must be used.
FIG. 2 illustrates an example structure of a dielectric patch antenna device using the dielectric. FIG. 2 shows a ground plane 21, a dielectric plate 22 mounted on the ground plane 21, an antenna element 23 provided on the dielectric plate 22, and a feed point 24 to the antenna element 23.
The size of the antenna element 23 is decreased by using the dielectric plate 22. It becomes possible to obtain high electromagnetic field radiation characteristics within a wide elevation angle range.
However, the antenna element size is decreased for the dielectric patch antenna device in FIG. 2. Compared to the air patch antenna device in FIG. 1, the antenna device gain greatly decreases. In addition, a loss due to the dielectric plate 22 further decreases the antenna device gain. As a result, the dielectric patch antenna device in FIG. 2 does not provide so high a radiation level toward a low elevation angle.
An object of the present invention is to provide a planar antenna device which satisfies both of electromagnetic field radiation characteristics over a wide elevation angle range including a low elevation angle direction and a high antenna device gain.
A planar antenna device according to the present invention comprises: a ground plane; a planar antenna element having a principal plane mounted above the ground plane; and a cavity, having an opening partially exposing the antenna element, placed on the ground plane in order to cover the entire antenna element contactlessly.
Preferred manners for the above-mentioned planar antenna device are as follows.
(1) A feed point for supplying power supply to the antenna element is further provided.
(2) An area of the opening is smaller than a size of the antenna element.
(3) The opening is placed substantially parallel to a principal plane of the antenna element.
(4) The antenna element is an air patch antenna element mounted above the ground plane separated by a spacer.
Another planar antenna device according to the present invention comprises a ground plane; a planar antenna element having a principal plane mounted above the ground plane; and a planar conductor placed substantially parallel to a principal plane of the antenna element and having an opening at substantially a center thereof.
According to the present invention, it is possible to provide excellent electromagnetic field radiation characteristics over a wide elevation angle range including a low elevation angle direction and a high antenna device gain only by adding a cavity to a conventional air patch antenna device without decreasing the antenna element size, thereby maintaining sufficiently high antenna device gain.
Further, the present invention eliminates the need to use a dielectric for obtaining a gain toward a low elevation angle. It is possible to maintain a high antenna device gain without decreasing an antenna device gain due to a dielectric loss.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.